In October 2002, with the country still reeling from the terrorist attacks of a year earlier, Americans were riveted and terrified by another outburst of violence. Over the course of three weeks, someone equipped with a high-powered rifle fired 15 times from a distance at people who were getting gas or mowing a lawn.

As with the similarly unfathomable anthrax mailings of fall 2001, initial suspicions that Al Qaeda was behind the Beltway sniper attacks proved to be unfounded. The investigation, headed by former Portland police chief Charles Moose, eventually apprehended 41-year-old John Allen Muhammad and 17-year-old Lee Boyd Malvo. The attacks have been the subject of two previous films (one made for the USA Network and one released straight to video), but “Blue Caprice” digs beyond the sensational headlines to try to figure out how this strange, fatal partnership was formed.

French director Alexandre Moors and screenwriter R.F.I. Porto devote most of their first feature to tracing the killers’ relationship, from an initial meeting on the Caribbean island of Antigua to Muhammad’s virtual adoption of Malvo while both lived in Tacoma, Wash. As Muhammad, a divorced father who seethes with anger toward his ex-wife and frustration at his own failings, Isaiah Washington gives a gripping performance, one that could signal his return to prominence. (Washington has been relegated to acting purgatory since his acrimonious, homophobia-tinged departure from TV’s “Grey’s Anatomy” in 2007.)

Even more impressive is young Tequan Richmond (TV’s “Everybody Hates Chris”) as the quiet, intense Malvo, a kid so desperate for a father figure in his life that he becomes putty in the hands of a killer. “Blue Caprice,” which takes its title from the vehicle they eventually use during the attacks, follows this doomed trajectory without judging or explaining their actions. Perhaps delving into the psychology of, especially, Muhammad would have been futile, but the result is a distancing coolness that sometimes wears thin.

Tim Blake Nelson and Joey Lauren Adams provide some perspective as a couple who know Muhammad and Malvo, but you still don’t have any more of an idea of what made them tick after the film than you did before. Then again, that’s sort of appropriate, since their deadly, impersonal actions are beyond explanation.